Physicists iron out lumps in quantum dots

Continuing to shrink the scale of electronics presents a host of problems, including the way surfaces interact with electrons. At the smallest scale, it's difficult to get a "ballistic" electron to follow a consistent path, something an international team of physicists hopes to solve.

In a paper published in Physical Review Letters (abstract), a group from the University of New South Wales, University of Oregon, Niels Bohr Institute and Cavendish Laboratory at Cambridge has demonstrated that removing dopants from a silicon substrate can help overcome the problem of unpredictable electron scattering at the quantum scale.

As they point out in their abstract, the issue is this: the electrostatic nature of dopants that provide the electrons for a device like a quantum dot lead to “unpredictable changes in the behavior of devices … each time they are cooled for use”. That unpredictability makes it hard to perform experiments with reproducible results.

“Impurities and defects in the semiconductor present a serious challenge” when working at the quantum scale, explained UNSW’s Professor Andrew Molich. Electrons passing across the surface of a quantum dot are scattered in a disordered fashion.

Solving this problem, the university explains has both theoretical and practical applications. Understanding how electrons’ wave-like nature affects transistor function is important as we try to shrink devices ever further; while more esoteric considerations include understanding “how classical chaos theory works in the quantum mechanical limit”, Molich explains.

While ultra-clean materials go some way to solving the problem, the electrostatic effect of the electrons in silicon doping has a “more subtle” effect on electron paths.

One aspect of the research was to demonstrate the magnitude of dopants’ effects. It had been assumed that surface irregularities, and the shape of the quantum device (whether it’s square or circular), were the most important, while the doping was insignificant. Repeated warming and cooling (towards zero Kelvin) of a quantum dot changed the electron paths each time, demonstrating the importance of the electrostatic effects.

UNSW’s Dr Andrew See, lead author of the paper, then demonstrated in his PhD thesis that removing the dopants meant electrons fired across the surface follow a much more predictable path.

“Ultimately, our work provides important insight into how to make better nanoscale electronic devices, ones where the properties are both more predictable and more consistent each time we use them,” Molich said. ®